Data recovery techniques in present-day communication equipment, such as high frequency modems, commonly make use of adaptive equalizers which are automatically and continually adjusted in an effort to reduce or substantially eliminate intersymbol interference caused by frequency dependent characteristics of the channel. In the case of a modem employing a two-sample per pulse equalizer in which the weighting coefficients are continually updated, modem performance is essentially independent of the relative timing between the transmitted symbol clock and the received symbol clock provided that the drift between the two clocks is sufficiently slow to permit the equalizer to track and the equalizer weight pattern is contained within the time span of the equalizer. Therefore, with sufficient clock stability, and a drift of no more than a few symbols during the time span of modem usage corresponding to that of a normal telephone conversation (less than twelve hours), this symbol rate drift between the transmitted and received clocks may be tolerable.
However, with the expanding development of modem usages, maintaining adequate relative transmitter and receiver symbol timing for considerably lenghty periods of time (as long as several months) requires some scheme of adjusting the received symbol clock to track the transmitter clock. Standard timing loops have been found to be undersirable in that they are either too slow to acquire, they contain too much timing jitter, or both. In place of using conventional timing loops, techniques have been developed whereby the gain coefficients of selected individual taps or stages of a transversal equalizer have been used to provide for carrier and timing control. For example, the U.S. Patents to Gibson, Re 28,638 (3,694,752), Yamamoto et al, 3,872,381 and Stuart 3,909,752 decribe timing recovery schemes wherein the gain coefficients at two respective points on opposite sides of the center point or main tap of a transversal equalizer are compared and used to control the recovered clock signal and/or carrier. In these systems, wherein the transmission rate is fairly low, the center tap of the equalizer may be used as the point of reference, since the largest weighting coefficients are predictably located at the center portion of the equalizer, and over the span of a conversation, any variation in the gain coefficient weighting pattern may only be on the order of one delay tap or stage, so that monitoring the gain coefficients of individual tap locations symmetrically disposed about the middle stage of the equalizer may provide satisfactory timing and carrier phase correction.
However, where the data transmission rate is considerably greater than the relatively low rates employed in the systems described in the above-referred to patents (4800 bauds per second in the patents to Gibson for example), the heaviest or largest weighting coefficients will not necessarily be located at the center of the equalizer and may be shifted substantially toward one end of the equalizer. Therefore, the systems described in the above patents are limited to low transmission rates and are non-workable for communication systems whose transmission rates are considerably higher. It goes without saying that using the weighting coefficients at individual taps immediately adjacent to either side of the center of the equalizer whose values are subject to substantial variation and may be shifted considerably from the location of the maximum weighting coefficients would make accurate symbol timing effectively unattainable.